RFR: 8305896: Alternative full GC forwarding [v29]

[Roman Kennke]

On Mon, 8 May 2023 05:10:11 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> src/hotspot/share/gc/shared/gc_globals.hpp line 698:
>> 
>>> 696:           constraint(GCCardSizeInBytesConstraintFunc,AtParse)               \
>>> 697:                                                                             \
>>> 698:   develop(bool, UseAltGCForwarding, false,                                  \
>> 
>> I don't think you can opt-in into `true` in release bits, if this flag is `develop` when the rest of Lilliput arrives. In release bits, all checks involving this flag would fold with `false`. Maybe that's the intent here, as it keeps the release performance at the baseline level, but this makes performance overhead estimations for this PR a bit hard :) [I'll hack the flag back to experimental for tests]
>
> If you need this for compact object headers then shouldn't this flag be experimental?

@tschatzl requested it to be develop *for this PR* and make it product/experimental in the compact-headers PR, because if the compact-headers PR doesn't make it into 21, then we'd have the slight performance hit of checking the flag (in a loop) in release builds for no benefit. See discussions above. I thought that is reasonable.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13582#discussion_r1187043439